Added fallback for gl-get-proc-address to use the cffi equivalent.

On MacOSX I ran into issues that it could not find certain function,  I traced it to the fact that there was no function to resolve the address of the opengl extension functions.

The cffi:foreign-symbol-pointer worked and because cffi is already used by cl-opengl I added it as the default fallback function.

For me this works. 

Kind regards,
Wim Oudshoorn.